class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int left=0;
        int right=arr.size()-1;
        int ret=-1;
        while(left<right)
        {
            int mid=left+(right-left+1)/2;
            if(arr[mid]>arr[mid-1])
            {
                left=mid;
            }
            if(arr[mid]<arr[mid-1])
            {
                right=mid-1;
            }
        }
        return left;
    }
};
